As shown in FIGS. 1 and 2, a hot-rolling composite roll 10 comprises an outer layer 1 coming into contact with a sheet to be rolled, and an inner layer 2 fused to an inner surface of the outer layer 1 and made of a different material from that of the outer layer 1. The inner layer 2 comprises a roll body portion 21 fused to the outer layer 1, and a driver-side shaft portion 22 and a follower-side shaft portion 23 integrally extending from both ends of the roll body portion 21. An end portion of the driver-side shaft portion 22 is integrally provided with a clutch portion 24 for conveying a driving torque. An end portion of the follower-side shaft portion 23 is integrally provided with a projection 25 necessary for handling the composite roll 10, etc. The clutch portion 24 has an end surface 24a and a pair of flat notch surfaces 24b, 24b engaging a driving means (not shown), and the projection 25 has an end surface 25a. The driver-side shaft portion 22 and the follower-side shaft portion 23 should be machined to have bearing portions, neck portions, etc.
Widely used as such a hot-rolling composite roll 10 is a composite roll having a composite structure comprising a centrifugally cast outer layer 1 having excellent wear resistance and failure resistance, and an inner layer 2 made of tough ductile cast iron and integrally fused to the outer layer 1. When damages such as wear and surface roughening, etc. occur in a surface layer of the outer layer 1 of the hot-rolling roll 10 by a thermal and mechanical load due to contact with a sheet to be rolled, a rolled sheet has poor surface quality. A composite roll 10 suffering large wear and surface roughening is exchanged to a composite roll 10 free from damages on an outer layer surface, and the outer layer 1 of the composite roll 10 taken out of a mill is re-polished to remove damages. The re-polished composite roll 10 is assembled in the mill again to be used for rolling. The frequent exchange of the composite rolls 10 results in frequent interruption of rolling, resulting in low productivity.
To make the frequency of the interruption of rolling as low as possible, attempts have been made to improve the wear resistance of the outer layer 1 coming into contact with a sheet to be rolled. As the wear resistance of the outer layer 1 has been improved to provide the composite roll 10 with a longer durable life, it has become important to improve the wear resistance of the clutch portion 24 coming into contact with a torque-conveying coupling. Extreme wear of the clutch portion 24 would make the composite roll 10 usable, even if the outer layer 1 were not worn.
As a hot-rolling composite roll comprising a clutch portion with improved wear resistance, JP 6-304612 A discloses a hot-rolling composite roll comprising an outer layer made of high-speed tool steel, and an inner layer and a shaft portion made of carbon steel containing 0.2-1.2% by weight of C or low-alloyed steel, a clutch portion made of spheroidal graphite cast iron comprising by weight 2.5-3.5% of C; 1.6-2.8% of Si, 0.3-0.6% of Mn, P<0.05%, S<0.03%, Ni<0.5%, Cr<0.2%, Mo<0.5%, and 0.02-0.05% of Mg, the balance being Fe and other inevitable components, and an area ratio of graphite being 5-15%, being integrally connected to an end of the shaft portion by casting. However, this clutch portion still has insufficient wear resistance. In addition, the end-to-end casting connection of the clutch portion to the shaft portion causes such a problem that foreign matter, etc. are likely introduced into an interface therebetween, resulting in casting defects. Further needed are flat cutting of casting-connected portions, setting a mold around portions to be casting-connected, and the melting and casting steps of spheroidal graphite cast iron for the clutch portion different from the inner layer, resulting in a higher production cost.
There is a further problem that if the driver-side shaft portion 22 and the follower-side shaft portion 23 were formed by the same hard material, the follower-side shaft portion 23 not required to be as hard as the driver-side shaft portion 22 would be unnecessarily hard, resulting in poor machinability.